Following violent, targeted attacks on 15 Division, the officers must piece together a past crime in order to find Oliver; the shooter's plan is revealed.
Andy finds out Marlo has been carrying out an unauthorized investigation into a pedophile; the drug sweep Traci is overseeing takes an unexpected turn when Sam leaves to help Marlo.
A routine drug bust turns into Traci's first murder case; Swarek, Oliver, Chris and Dov spend a weekend at Oliver's cabin; Andy and Sam both have confessions to make.
When the investigation of a missing girl is linked to a past case, Detective Luke Callaghan comes back to help with search; Traci realizes she needs to get past Jerry's death; Sam and Andy face their past.
When an arrest reveals that drugs contaminated with a deadly bacteria have hit the street, the squad rushes to control the situation; Marlo reveals a shocking secret; Dov suspects Chloe of hiding secrets.
Andy and Officer Cruz butt heads when a disturbed woman forces them to make a crucial decision; an armed burglar is sought after a string of ATM robberies; Dov and Chloe have to work together.
On her first day back, Andy is forced to work a bank robbery case with newly promoted Swarek; a distraught woman gives Nick perspective on his life with Gail; a bombshell is dropped on Noelle.
Nick and Andy go missing while working under cover to infiltrate a meth pipeline; Swarek and the other officers find themselves in the middle of a rescue operation; Andy works to deal with all the change going on around her.